Optical fibers can be used as distributed optical sensors in many applications, for instance, to measure strain or temperature along the fiber, or determine the three-dimensional fiber shape from simultaneous bend, twist, and axial-strain measurements. Fiber shape sensing is useful, for example, in robotic surgery, where collocating a robotically operated surgical tool with the end of the shape-sensing optical fiber allows to precisely determine the location of the tool within the patient's body based on a measurement of the three-dimensional fiber shape. To use an optical fiber as a sensor, the fiber is interrogated by sending light down the fiber and measuring reflections received from various locations along the fiber; the spectral characteristics of the reflected light generally provide information about local fiber properties, such as local temperature or strain. For the purpose of creating a well-defined spectral reflection profile that facilitates such measurements, optical fiber cores are often inscribed with fiber Bragg gratings variations in the refractive index of the core that form a distributed. Bragg reflector. For periodic refractive-index variations, the grating exhibits a narrow reflection band at a wavelength that is twice the periodicity of the grating. Fiber Bragg gratings can be created by exposing a photosensitive fiber core (e.g., made of germanium-doped silica) to intense ultraviolet (UV) light, causing index changes in the exposed regions that depend on the intensity and duration of the exposure. The desired pattern of index variation can be achieved by creating a corresponding intensity pattern of the UV light, e.g., using two-beam interference at or near the fiber core, or a light modulation mask (or “photomask”) placed between the UV light source (generally a laser) and the fiber.
Optical fibers are often surrounded by a protective coating that prevents, e.g., scratches and other damage to the surface of the fiber cladding. Since many industry-standard coatings do not transmit UV light, the process for inscribing fiber Bragg gratings used to involve stripping a pre-fabricated coated fiber, and recoating the fiber following the inscription. The increased cost and risk of fiber damage that are associated with these added manufacturing steps spurred the use of other formulations that are at least partially UV transparent, such as silicone. In conjunction with methods to increase the photosensitivity of the fiber core, such as boron co-doping, these more UV-transparent coatings allow fiber Bragg gratings to be written through the coating. The fiber coatings may, however, get scratched, and such scratches can partially block the UV light beam used to inscribe the grating, distorting the grating or preventing its creation.